pinctrl: mediatek: assign functions to configure pin bias on MT7986

Assign bias_disable_get/set and bias_get/set functions to allow
configuring pin bias on MT7986.

Fixes: 2c58d8dc ("pinctrl: mediatek: add pull_type attribute for mediatek MT7986 SoC")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/47f72372354312a839b9337e09476aadcc206e8b.1692327317.git.daniel@makrotopia.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>